Effects of urbanization and environmental heterogeneity on fish assemblages in small streams

Abstract The structure of freshwater assemblages may be driven directly by urbanization or indirectly by a reduction in environmental heterogeneity (EH). Disentangling the effects of urbanization and EH requires uncorrelated proxies of each of these factors. We assessed the effects of the degree of urbanization and EH on the structure of fish assemblages. We sampled fish in 45 streams located in the urban area of Cuiabá. We assessed the effects of urbanization and EH on rarefied fish species richness (Srarefied), the local contribution to beta diversity (LCBD), and composition with linear models and distance-based redundancy analysis. Our indexes of urbanization and EH were not correlated. We found that both Srarefied and the LCBD decreased with an increasing degree of urbanization, but were not associated with EH. We also noted that few native fish species abundances were associated with the EH. Serrapinnus microdon, S. calliurus, Hemigrammus tridens, and Astyanax lacustris were abundant in streams with a lower degree of urbanization. The non-native Poecilia reticulata was more abundant in streams with a higher degree of urbanization. Our results highlight that urbanization leads in negative impacts on fish assemblages, such as decreases in diversity and the dominance of non-native species.

摘要 淡水生物群落的结构既可能直接受城市化驱动，也可通过环境异质性（environmental heterogeneity, EH）的降低而间接受到影响。若要厘清城市化与EH的作用效应，需采用二者互不相关的替代指标。本研究评估了城市化程度与EH对鱼类群落结构的影响。我们对库亚巴（Cuiabá）城区内的45条溪流开展了鱼类采样，借助线性模型与基于距离的冗余分析（distance-based redundancy analysis），探究了城市化程度与EH对稀疏化鱼类物种丰富度（rarefied fish species richness, Srarefied）、群落对β多样性的本地贡献（local contribution to beta diversity, LCBD）以及群落组成的影响。

本研究采用的城市化程度与EH指标并无相关性。结果显示，随着城市化程度提升，稀疏化鱼类物种丰富度与LCBD均呈下降趋势，但二者与EH并无显著关联。此外还发现，仅少数本土鱼类物种的丰度与EH存在相关性。Serrapinnus microdon、S. calliurus、Hemigrammus tridens以及Astyanax lacustris在城市化程度较低的溪流中种群丰度更高；而外来物种Poecilia reticulata在城市化程度较高的溪流中种群丰度更高。

本研究结果凸显了城市化对鱼类群落的负面影响，包括物种多样性下降以及外来物种占据优势种群地位。